A wellbore is drilled by a downhole tool having a drill bit coupled to a lower end portion thereof. The drill bit drills the wellbore to a first or “pilot hole” diameter. The downhole tool may include an underreamer coupled thereto and positioned above (e.g., 15 m-45 m above) the drill bit for increasing the diameter of the wellbore from the pilot hole diameter to a second diameter. The underreamer includes a body having one or more cutter blocks movably coupled thereto that transition from a retracted state to an expanded state. In the retracted state, the cutter blocks are folded into the body of the underreamer such that the cutter blocks are positioned radially-inward from the surrounding casing or wellbore wall. In the expanded state, the cutter blocks move radially-outward and into contact with the wellbore wall. The cutter blocks are then used to cut or grind the wall of the wellbore to increase the diameter thereof.
The underreamer may be in the expanded state as the drill bit drills the wellbore. As the underreamer is positioned above the drill bit, the portion of the formation surrounding the drill bit oftentimes has a different hardness than the portion of the formation surrounding the underreamer. For example, the portion of the formation surrounding the drill bit may be softer than the portion of the formation surrounding the underreamer. As a result, the drill bit has a greater rate of penetration “ROP” than the underreamer (i.e., the drill bit is able to drill faster than the underreamer is able to ream). This causes the underreamer to wear down as the drill bit “pulls” the underreamer through the harder portion of the formation at a rate that is faster than optimal. What is needed, therefore, is a system and method for weakening the walls of the wellbore prior to increasing the diameter of the wellbore with the underreamer.